Pendimethalin is a dinitroaniline herbicide having chemical formula [n-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine). It was first registered for use in the United States in 1974. It is a selective herbicide which controls certain broadleaf weeds and grassy weed species in crop and non-crop areas. It is applied to soil preplant, preemergence, and postemergence with ground and aerial equipment.
Pendimethalin is an orange yellow crystalline solid with a melting point of 54-58° C. It is soluble in chlorinated hydrocarbons and aromatic solvents such as methylene chloride, acetone and xylene. Pendimethalin is stable under acidic and alkaline conditions.
Pendimethalin is currently available in various different agrochemical formulation types such as emulsifiable concentrate (21.9% to 42.3%), liquid (34.4%), granular (0.7% to 2.0%), soluble concentrate/liquid (22.0%), water dispersible granules, dry flowable (up to 60.0%), capsule suspension and wettable powder (50.0%). Thus, there are many formulation choices available to a formulator setting out to prepare a desired formulation of pendimethalin.
The different formulations applicable to an agrochemical differ in their effectiveness on the desired weeds, effects the tolerance of the turf and ornamentals to the herbicide, differ in their cost advantages, differ in the drift potential of the herbicide and also differ in the ease of application and compatibility with the application equipment. Another challenge to the skilled formulator is the selection of an appropriate surfactant. Surfactants are known to increase the rate of absorption of the herbicide in the weedy species but also increase the potential for plant injury to the desirable plants during herbicide drift. Yet another challenge before a skilled formulator setting out to obtain a formulation containing pendimethalin is selecting either a granular or a sprayable formulation.
Pendimethalin has been conventionally available in both granular and sprayable forms, which may differ in the degree of weed control. However, it is often advantageous to obtain a sprayable formulation containing certain herbicides due to inherent advantages of a sprayable formulation.
The granular formulations exhibit relatively low foliar absorption because most of the applied granules fall through the leaf canopy to the soil below. In contrast, sprayable products achieve good coverage and adhere better to the foliage, providing relatively good weed control. It has also been observed that a granular product comprises a greater relative amount of the inert ingredients comparative to a sprayable formulation. Thus, the amount of the formulated product to deliver the same dosage of the active ingredient is much greater with granules resulting in higher shipping and packaging costs.
An advantage of the sprayable formulation over granules is more uniform application achieved with the sprayable formulations. The granular formulations are usually difficult to apply uniformly, especially those that contain a relatively high concentration of active ingredient. Thus, it is more often advantageous to formulate sprayable formulations of certain agrochemicals.
However, phytotoxicity of some of the sprayable formulations including pendimethalin has been reported. It is desirable to provide formulations containing pendimethalin that do not display or reduces the extent of phytotoxicity.
Another challenge that relates to dinitroaniline herbicides, and pendimethalin in particular, is the potential for staining the sidewalks and other locations where the herbicide is applied. The active ingredient herbicides of the dinitroaniline class have a yellowish or yellowish-orange color. It is further known that granular formulations often do not stain badly, whereas a liquid formulation may cause more serious staining. Moreover, granules are easy to sweep or blow from concrete surfaces, whereas overspray need to be washed off before it dries.
Thus, it is desirable to provide sprayable formulations containing pendimethalin that substantially reduce the incidence of staining.
U.S. Pat. No. 4,871,392, discusses under the background thereof, that pendimethalin is known to exist in polymorphic forms as orange and yellow crystals. This patent further discusses that pendimethalin is a pesticide that is difficult to formulate owing partly due to the unique staining problems associated with it. This patent further discusses that the presence of pendimethalin in the orange macrocrystal form results in large elongated crystals in final formulations. Moreover, when pendimethalin in the orange macrocrystal form is found in compositions, very large, elongated crystals (about 3000 microns in length) appear in final product, resulting in instability, difficulty in processing and unreliability of usage apart from the severe staining further compounded by a severe clogging of nozzles.
U.S. Pat. No. 5,705,174 discloses microencapsulated pendimethalin formulations i.e. an aqueous concentrate composition of pendimethalin particles which are encapsulated by a polymeric wall material, which show a reduced tendency to form large crystals. These compositions also have improved storage stability. Unfortunately, it has been seen that microencapsulation of pendimethalin tends to slow the release of the active ingredient. U.S. '174 teaches an aqueous capsule suspension formulation of pendimethalin, which contains about 456 grams of active ingredient per liter. The encapsulation of pendimethalin allows for the elimination of organic solvents in the product. The elimination of organic solvents reduces odor, staining to a certain extent, volatility and surface residue adhesion as compared to existing emulsifiable concentrate formulations of pendimethalin. Moreover, the microencapsulated pendimethalin formulation is stable under conditions of freezing and thawing and is compatible with liquid and dry fertilizer. However, the existing problem of staining, phytotoxicity and corrosiveness is not adequately addressed by the disclosure of U.S. Pat. No. 5,705,174.
Moreover, this patent essentially teaches a microencapsulated formulation of pendimethalin containing an inorganic salt, which is added prior to microcapsule formation. It is disclosed that the addition of an inorganic salt or mixtures thereof prior to the microcapsule formation provides a visibly cleaner microcapsule as more of the colored active ingredient is encapsulated, which is therefore unable to stain the external surfaces of the microcapsules.
These microcapsules are also stated to be less prone to breakage than the microcapsules prepared by the conventional methods. Examples of inorganic salts taught by this patent are sodium chloride, calcium chloride, potassium chloride, sodium nitrate, magnesium sulfate and/or sodium sulfate.
However, these inorganic salts are not without additional disadvantages. The use of an inorganic salt such as sodium chloride even to the extent of 0.1-0.5% has been shown to aggravate the already existing phytotoxicity of pendimethalin which is evident from “Sodium and chloride ions contribute synergistically to salt toxicity in wheat,” Biologia Plantarium, 37 (2); 265-271, 1995. Here, Martin et al., studied the effects of supplying excess mineral salts, involving sodium as a cation and a range of counter-anions, including chloride on the growth and photosynthetic capacity of a salt susceptible bread wheat. It was found that the synergistic effect of sodium and other alkali and alkaline earth metals with chlorine shows that neither of these ions is alone responsible for the salt stress induced damage in plants.
Moreover, these inorganic salts dissociate and/or dissolve in water and increase the hardness of water. The increased water hardness further reduces the temperature and suspension stability of microencapsulated pendimethalin. Moreover, the polymeric capsule wall of the disclosed microencapsulated pendimethalin is still susceptible to breakage to an appreciable extent. Thus, the problem of staining continues to plague the existing state of the art.
A need remains in the art for microencapsulated formulations of pendimethalin having improved non-staining property with reduced phytotoxicity problems.
Clomazone is the common name for the herbicide 2-(2-chlorophenyl) methyl-4,4-dimethyl-3-isoxazolinone. It is a colorless to light brown and viscous liquid above room temperature, which forms a white crystalline solid when cooled. It is not flammable in nature.
Clomazone has the following chemical structure:

Clomazone is a highly effective herbicide, but unfortunately is also highly volatile. The amount of clomazone applied to the soil in a target area may move to adjacent areas and cause discoloration. This discoloration is typically in the form of whitening or some degree of bleaching, of a variety of crops, trees, or decorative plants. This bleaching, which is also indicative of the mode of action of the herbicide, may be temporary when plants are exposed to sufficiently low concentrations. However, bleaching of the crops, trees or decorative plants is undesirable even when it does not result in the destruction of the plant.
Clomazone is a broad spectrum herbicide used for control of annual grasses and broadleaf weeds in cotton, peas, pumpkins, vegetables, sweet potatoes, tobacco, winter squash and fallow wheat fields. It can be applied early preplant, preemergent or preplant-incorporated depending on the crop, geographical area and timing. Because clomazone is an inhibitor of plant pigments, users must exercise caution to avoid drift or vapors which may cause bleaching damage to non-target foliage.
Clomazone is relatively stable to degradation by UV light. It is highly volatile and can drift during or after application, causing damage to sensitive, non-target plants such as ornamental trees and shrubs, roses, small grains, alfalfa, sunflowers, and vegetable crops. Clomazone is slightly soluble in water, but it has a moderate tendency to adsorb to soil particles. It therefore has a low to moderate potential to contaminate groundwater.
Hitherto, a non-encapsulated emulsifiable concentrated formulation of clomazone has been available. However, upon application of the conventional emulsifiable concentrated formulation, the sensitive plants surrounding the intended targets of application displayed varying degrees of whitening due to the high volatility of clomazone. Thus, an encapsulated formulation of clomazone was desirable which was believed to be capable of reducing the volatility of clomazone and improve the active component delivery to the targeted plants.
Attempts to prepare formulations of encapsulated clomazone by encapsulating clomazone in polyurea and polyamide polymeric shells frequently resulted in formulations that not only gave little or no reduction in volatility, but had poor physical characteristics such as undesirable agglomeration of the capsules or separation of phases or breaking of the capsule wall on spray application which results into failure to achieve the volatility reduction. It was thus desirable to provide a herbicidal formulation having an improved plasticity of the polymeric shell wall to reach an acceptable release rate of the active ingredient clomazone. It is believed that an improvement in plasticity of the polymeric shell wall would substantially reduce the permeation of the shell wall to the active ingredient and possibly limit the breakage of the capsule wall on spray application, which would consequently achieve a substantial reduction in volatility of the formulation.
Another challenge during the encapsulation of clomazone had been its relatively higher water solubility. The known encapsulation methods involve a reaction between an aqueous phase and an organic phase. It was found that the low/mild solubility of clomazone in water did result into poorly defined droplets and also increased the amount of the free active ingredient in the aqueous phase. An increased amount of clomazone in the aqueous phase could contribute to an increased initial “burst effect” administration of clomazone thereby aggravating the risk of plant phytotoxicity and off-target injury due to the volatility of the free clomazone.
U.S. Pat. No. 5,583,090 is directed to a sprayable herbicidal formulation comprising an aqueous liquid having suspended therein a multitude of solid microcapsules having a capsule wall of a porous polymer encapsulating clomazone dissolved in a high boiling inert organic solvent.
U.S. Pat. No. 5,597,780 teaches a process for preparing herbicidally effective formulations of clomazone by microencapsulating clomazone by interfacial polymerization reaction between an aqueous phase and an organic phase. The organic phase essentially comprises a hydrocarbon solvent.
These patents disclose that when the formulations taught therein are sprayed onto one plot containing vegetation, vapor transfer of the herbicide to a nearby plot containing vegetation is effectively suppressed without substantial sacrifice of herbicidal efficacy of the herbicide in the plot to which the spray is applied. However, the problem of reducing the permeability of the polymeric shell wall to the active ingredient consequently limiting the rupture of the capsule wall on spray application and substantially reducing the volatility of the microencapsulated formulation of clomazone still remains a problem. Further, this problem continues to remain irrespective of the chemical nature of the polymeric shell wall such as a polyamide, polyurea, polyurethane, polycarbonate, melamine resin, melamine urea resin, gelatine/gum arabic or cross linked or non-crosslinked combinations thereof. Moreover, the use of an organic solvent is not always desirable.
The regulatory bodies throughout the world are slowly imposing stringent restrictions on the quantity and choice of solvents in a formulation. Typical organic solvents used in pesticide formulations usually have a low boiling point and evaporate easily or can be removed by distillation, leaving the dissolved substance behind. Solvents are usually clear and colorless liquids and many have a characteristic odor.
The low vapor pressure of traditional solvents generally makes them hazardous because these solvents evaporate easily into the air exposing the factory workers to inhalation hazards. Additionally, some solvents add to damage of the earth's atmosphere as they eventually oxidize and create carbon dioxide, a green house gas with potential impact on global warming.
Most organic solvents have a lower density than water, which means they are lighter and will form a separate layer on top of water. Further, most organic solvents are flammable or highly flammable, depending on their volatility. Mixtures of solvent vapors and air can explode. Solvent vapors are heavier than air, they will sink to the bottom and can travel large distances nearly undiluted. Many solvents can lead to a sudden loss of consciousness if inhaled in large amounts.
A major pathway leading to adverse health effects arises from spills or leaks of solvents that reach the underlying soil. Since solvents readily migrate substantial distances, the creation of widespread soil contamination is not uncommon. This is particularly a health risk if aquifers are contaminated. Some solvents including chloroform and benzene are carcinogenic. Many others can damage internal organs like the liver, the kidneys, or the brain.
In agrochemical formulations, the organic solvents are usually present in the smaller and/or greater amount depending upon the type of formulation and agrochemicals. The use of routine organic solvents poses problems while using, manufacturing, storing, transporting the solvents and the products comprising them.
In general, agrochemical formulations especially liquid form comprises either inorganic or organic solvents. Most of the known organic solvents known in the art are non-biodegradable and highly flammable. Organic solvents-based agrochemical formulations generally use a solvent that is preferably water-immiscible to dissolve the active component completely and produces a clear homogenous liquid free from extraneous matter. Alternatively, organic solvents typically have a low flash point, are non-biodegradable, cause skin irritation and possess medium or high evaporation rate etc., but provide a clear homogenous liquid. The known agrochemical compositions further include at least a surfactant wherein the performance and dosage of the included surfactant is based on the active content and solvent in the formulation, type of active ingredient, and solubility of the active ingredient in the solvent and the required emulsion performance of the final product. In many cases, the emulsion performance shown by organic solvent can be superior to that of inorganic solvents. However, certain challenges exist with solvent usage because the solvent used are non-biodegradable and also require a large quantity of the surfactant to emulsify the formulation during dilution, prior to the application on crops. With organic solvents, during manufacturing, packing, storage, transport and use the risk of skin irritation, non biodegradability, fire hazard, air and soil pollution exists.
Moreover, the regulatory bodies around the world are now considering the public disclosure of all pesticide inert ingredients including the solvents. These regulatory bodies are making it mandatory to identify all the inert ingredients including solvents etc. on the product label. Thus, a formulation which substantially reduces or completely eliminates the need of an organic solvent to be present within the formulation and is thus completely biodegradable would be a highly desirable formulation from a regulatory and customer acceptance point of view.
Accordingly, there is a need in the art for an agrochemical formulation that is devoid of an organic solvent or that includes an organic solvent in minimum required quantities. The present invention described herein provides such an agrochemical formulation.
It is generally desirable to prepare formulations of agrochemicals having minimum volatility to reduce the incidence of off-site injury in order to avoid any unintended pesticidal activity. The volatility of agrochemical formulations is known to cause various off-site unintended injuries. Thus, it is another challenge in the art to prepare storage stable formulations of agrochemicals having a substantially reduced volatility without compromising the other desirable properties of the formulation.
The use of herbicide combinations is a widespread and documented practice in the agricultural community. Herbicidal combinations offer significant advantages over individual applications including improved and extended weed control, reduced herbicide rates and application costs, shorter contact times for improved results in flowing water, less stringent use restrictions, improved selectivity, improved spectrum of weeds controlled, reduced cost and reduced residue problems. However, identifying appropriate herbicide application rates and combinations is essential to achieve synergistic weed control. The different individual problems discussed above for pendimethalin and clomazone make it further difficult for a skilled formulator to co-formulate the two herbicides in a manner that simultaneously reduces the staining in pendimethalin and volatility in clomazone.
The article Effect of 18% pendimethalin+clomazone WP against weeds in transplanted Rice, XU Xiu-jie, Zhang Xiang-quan (Jilin Yiheng Pesticide Co. Ltd., Jilin Gongzhuling 136100, China; Jilin Ruiye Pesticide Co. Ltd., Jilin Gongzhuling 136100, China) evaluated the control of 18% pendimethalin+clomazone WP against weeds in transplanted rice. It was found that excellent weed control was achieved when 18% pendimethalin+Clomazone WP was applied at 5 to 7 days after transplanting and with the dosage of 0.175-0.351 kg/hm2. This reference does not address the staining problem associated with pendimethalin and volatility and associated off-site injury with clomazone, particularly when the two herbicides are encapsulated together.
There is thus a need in the art for an encapsulated formulation of pendimethalin and clomazone that overcomes the aforesaid problems in the art.